Resistor element



June 22, 1937. G, M EHLERS 2,084,840

RES ISTOR ELEMENT Filed Dec. 4, 195s IN VENTOR.

16M, AM/l ATTORNEY Patented June 22, 1937 UNITED lSTATES PATENT GFFICE RESISTOR ELEMENT Application December 4, 1933, Serial No. 700,872

13 Claims.

This invention relates to improvements in socalled xed resistor elements particularly applicable for radio, communication and allied electrical circuits and to a method of and apparatus for making the elements.

This application is in part a continuation of my pending application for Electrical resistor elements, Serial No. 462,976, filed June 23, i930.

One object of my invention is to provide a rem sistor element having an electrical conducting path in the form or" a core of relatively small cross-sectional area, surrounded by and in intimate contact with a rigid insulating jacket of considerably larger cross-sectional area to in- 15 crease the mechanical strength of the resultant structure and to protect the conducting path from moisture and accidental contacts, as Well as to provide for the removal of heat generated in the core when in use.

It is Well known in the art of making cornposition resistors, consisting of an electrical conducting material mixed with non-conducting material, that the diiculty of obtaining uniform reproduction of a desired resistance value increases very greatly the higher the desired resistance value. This is due to the fact that as the per cent of electrical conducting material in the mixture decreases below a certain value, the specic resistance of the mixture increases extremely rapidly witheach small decrease in electrical conducting material content, so that it becomes extremely diicult to control a desired resistance value lying in this range of specic resistance. In my resistor, however, the specific resistance of the core mixture can be kept low enough to keep it out of this critical range and the desired resistance value can be obtained by making the core of small cross-sectional area.

Such a resistor of small cross-sectional area, would not have sufficient mechanical strength to withstand handling, nor would it have by itself sufiicient surface area to dissipate the heat generated in it by the passage of an electric current. In my resistor these difficulties are overcome by surrounding the resistance core of small cross-section with a strong insulator jacket of considerably large cross-section and in intimate contact with the surface of the core, whereby the latter is given mechanical support and any heat generated is conducted directly to the surrounding jacket and readily dissipated from its larger surface area.'

Another object of my invention is to provide a resistor having a core of conducting material of (Cl. 20L-76) high resistance and superior electrical permanence.

Another object of my invention is to provide a resistor element as described in which the core has substantially the same drying and ring shrinkage factor and coefficient of thermal ex pansion as the jacket, whereby drying before ring, cooling from the firing temperature in the manufacture of the element and subsequent heating and cooling of the element in service, will not cause destructive stresses between the core and the jacket to crack them or disrupt their intimate contact. To accomplish this object, I compound the core and the jacket of materials having these properties, and also having the property ci vitri tying into a strong integral mass in the firing and subsequent cooling of the element in the process of its manufacture. This provides an intimate connection between the core and the jacket for strength, for heat dissipation, and for the equalization of strains and stresses produced in the element during its manufacture and use. I rind that ceramic materials give these results and it is a further object of my invention to compound the core and the jacket of these materials as hereinafter set forth. A ceramic material for the core provides a vitriiied insulating matrix for the electrical conducting material for the core and holds it evenly distributed in place in the core. A ceramic material for the jacket provides a strong ceramic insulating jacket about the core giving it adequate protection and support.

Another object of my invention is to provide the conducting particles for the core of such nneness that when distributed through the holding matrix of the core there will be a minimum of contact resistance between adjacent particles, whereby the usual source of minute changes of resistance and consequent microphonic noise will be minimized, if not entirely eliminated as desired for radio or communication circuits.

As to the method of my invention, it is an object to make the resistor element by simultaneously extruding the materials composing the core and the jacket, While in a plastic form, from a die or other apparatus so constructed that the materials as they are expressed from the die will be simultaneously shaped to the form required for the core and the jacket and be disposed in assembled relation one within the other, and in intimate contact ready for the subsequent treatment required for the manufacture of the finished element.

This process of simultaneous extrusion of the core and the jacket materials in assembled relation is new in the art and involves careful selection of the materials for both the core and the jacket, so that they will extrude satisfactorily, Will vitrify and will not crack in the drying or firing of the extruded piece. To satisfy these conditions, I find that not only the kind of materials for the core and the jacket must be correctly chosen, but also the grain sizes of the materials used is of considerable importance as hereinafter more fully set forth.

A further object of my invention is to provide the resistor element with a glazed coating on the outer surface of the jacket, said coating comprising an enamel frit, preferably sprayed on the jacket as the extruded piece leaves the die. This coating forms a glaze which seals the jacket against the admission of oxygen and protects the carbonaceous electrical conducting material of the core from oxidation at the high temperature to which the piece is subjected during the period of vitriiication of the jacket and core ceramic materials.

As to the apparatus of my invention, it is an object to provide a die so designed and constructed that the core and the jacket materials may be simultaneously extruded in one assembled piece.

The invention consists further in the matters hereinafter described and claimed.

In the accompanying drawing:-

Fig 1 is a perspective view of a resistor element constructed in accordance with my invention;

Fig. 2 is a longitudinal sectional view of the element;

Figs. 3 and 4 are transversesectionalviews taken on lines 3-3 and 4-4 respectively, of Fig. 2;

Fig. 5 illustrates a modified form of terminal construction to be hereinafter described;

Fig. 6 shows a method of and apparatus for making the element; and l Fig. 'I is an enlarged sectional View illustrating diagrammatically the manner in which the core and the jacket of the element are joined together in a vitriiied integralmass.

As shown in the drawing, the resistor element of my invention comprises a core I and a surrounding jacket 2. The core provides an electrical conducting path for the element when in use and is composed of a mixture containing the de-` sired high resistance material. The jacket 2 provides an insulator and support for the core and is made of the desired insulating and mechanically strong material. Ihe core and the jacket in the making of the resistor element are vitriiied into a strong integral mass as hereinafter described.

The core l is in lthe form of a solid rod and the jacket 2 is in the form of a tube having a solid surrounding Wall. The construction is such that the rod forming the core is in intimate contact with the bore of the tube and is vitrified thereto to provide an integral connection between them. The core and the jacket are co-extensive in length so that the ends of the core are at or adjacent the ends of the jacket, thereby making the core accessible for connecting it in an electrical circuit. Contact is made with the core at the ends of the jacket by the use of suitable terminal means, which, as shown in the drawing, are in the form of metal caps 3, 3 iltting over the ends of the jacket and in electrical contact with the ends of the core. These caps are preferably in the form of metal coatings applied over the ends of the jacket and the core by a metal spray process which secures an intimate electrical Contact between the core and the caps. In this form, the resistor element could be used in an electrical circuit by inserting it between and in contact with spring or other metal contact members in the circuit.

In Figs. 1 and 2, however, I have shown lead wires 4, 4 connected to the terminal caps or coatings 3 at the ends of the element for circuit connecting purposes. The wires are preferably soldered to the caps as by dipping the ends of the element as provided by the caps in molten solder after the caps have been applied and the wires have been wound or wrapped about the caps one or more turns. 'I'his provides soldered coatings 5, 5 over the metal caps 3 to protect them, and also secures the wires to the caps. The latter fit tightly over the ends of the jacket and are secure against removal.

In the form of terminal construction shown in Fig. 5, the wires are directly connected with the core I, the wires at the opposite ends of the resistor element extending into the material of the core through apertures in the metal caps. Only one end of the element is shown in Fig. 5. There the wire 4a is shown extending into the core l through an aperture 6 in the adjacent metal cap 3. The wire ts in a bore la in the end of the core and is secured to the metal cap by a soldering coating 5a applied over the outer side of the cap. A similar arrangement is provided at the opposite end of the element.

To give mechanical strength to the resistor element, the cross-sectional area of the jacket 2 is considerably larger than that of the core l. This increased wall thickness of the jacket also protects the core from moisture and accidental electrical contacts and additionally facilitates the removal of the heat generated in the core in the use of the resistor element. The relative cross-sectional dimensions of the core and the jacket are illustrated in Fig. 2 and to better advantage in the cross-sectional views, Figs. 3 and 4.

The materials employed for the core and the jacket are selected to give the following results.

.To render the mixture composing the core and the jacket suiciently plastic to facilitate their simultaneous extrusion from a die or other form in the making of the element. To enable the core and the jacket to be vitrified into a strong integral mass in the ring and subsequent cooling of the extruded piece to give the element the desired mechanical strength, thermal shock proof resistance and thermal conductivity. To provide the jacket in the form of a strong vitriiied mass to support a core of small cross-section to give the desired resistance value. To provide the core in the form of a vitriiled matrix to hold the electrical conducting material in the core in place, and to obtain the proper drying shrinkage of the core relative to the jacket and substantially the same thermal co-eiiicients of expansion between the core and the jacket to prevent cracking of the core or the jacket or the loosening of their connection in the heating and cooling of the element in its process of manufacture and also when in service. It is to be understood that the material of the core in its unfinished state before firing may not be at all electrically conducting but becomes conducting after being fired. The extruded rods even in their dried state before Ill-ing have avery much higher electrical resistance than they have in their finished fired state. The purpose of the relatively small diameter core herein mentioned is that more desirable electrical characteristics can be controllably obtained with a core having a high proportion of conducting material in its composition. Since this gives the composition a low specific resistance, it is necessary to provide a core of relatively small cross sectional area to get the desired high resistance values in the finished resistors.

Certain ceramic materials have the properties which give these results. The ceramic material selected for the core need not necessarily be the same as the ceramic material selected for the jacket, but it is essential that the core and the jacket both contain ceramic materials to obtain their vitrification and the matching or equalization of the two bodies in shrinkage and thermal co-eiiicients of expansion. It is also essential to maintain a certain grain size relation between the materials composing the core and those composing the jacket. The core of the desired characteristics is composed of a relatively coarse grain, non-plastic, refractory ceramic lapproximately 100 mesh), a fine grain plastic ceramic, sufciently fine to make a plastic colloidal suspension, and an electric conducting material such as finely divided graphite or carbon. The jacket of the desired characteristics is composed or a relatively iine grain size, non-plastic refrac tory ceramic (approximately 200 mesh or finer) fine plastic ceramic clays, and a small quantity ci some refractory material of good thermal conductivity. The finely divided ceramics in the core and the jacket render their mixtures suiiiciently plastic to facilitate their extrusion in the making of the element, as well as serving as binders for the nen-plastic ingredientsof the core and the jacket, namely, the non-plastic ingredients in the jacket and the refractory ceramic and the conducting material in the core. The ceramic mixture of the jacket is much more complex than the ceramic mixture of the core. rI'he purpose of this complex mixture is to obtain a vitrified body, which will have the desired mechanical strength, thermal heat shock resistance and thermal conductivity. The choice of these ceramic materials, of course, also governs the thermal co-eilicients of expansion of the vitriiied ceramic. The choice oi a larger grain size for the refractory in the core material than in the jacket material is important in obtaining the proper drying shrinkage oi the core relatively to the jacket. The inclusion of the fine plastic ceramics in the core and the jacket materials renders them suflciently plastic for extruding these materials in the making of the element. The ceramics in the two bodies fuse together in the making of the element. The ceramics chosen for the core and the jacket will be those best suited to accomplish the results herein set forth. I do not wish to be limited to any particular or designated ceramics or other materials except as herein claimed, it being within the contemplation and scope of my invention to make the resistor element of these materials which will give the results herein described and will provide the desired matrices.

The method of and apparatus for making the resistor element as shown in Fig. 6 enables the core I and the jacket 2 to be produced simultaneously in their assembled relation, one Within the other and in intimate contact, from the materials composing them. The apparatus comprises a die having a pair of concentric cylinders 1, 8 and similarly disposed extruding nozzles 9 and II) at their discharge ends. The inner cylinder I is loaded with the desired plastic mixture for the core. The outer cylinder 8 is loaded with the desired plastic mixture for the jacket. These materials are then subjected to pressure, as by pistons or plungers Il, I2 operating in the respective cylinders and the contents of the cylinders are forced out of the same through the nozzles in the form of a continuous extruded piece or rod I3, the core material being within the jacket material and in intimate contact therewith, as shown. 'Ihe rod has the size and shape required for the core and the jacket for the finished element.

As the rod I3 is extruded from the nozzles, it has been found desirable to coat the outer surface of the rod with a coating I4 of a glaze enamel having a lower melting point than the vitrifying temperature of the ceramic materials of the core and the jacket. This is not an indispensable coating, but has been found desirable as hereinafter explained. The enamel coating I4, called an enamel frit, may be applied to the outer surface of the rod as it is extruded from the nozzles in any desired manner. A spraying process is found desirable for this purpose, spray nozzles I5, l5 of the character required being used, as illustrated in Fig. 6. These nozzles are so positioned and arranged about the path of the rod as it is extruded from the die that an enamel coating is applied simultaneously about the entire outer surface of the rod as it leaves the die. Three nozzles may be used. They are disposed in substantially right angular relation to the axis of the extruded piece and in equal spaced relation about the same. This arrangement as shown in Fig. 6 is by Way of an example, it being understood that the nozzles will be so arranged and disposed to coat the entire outer surface of the rod with the enamel coating M simultaneously and as the rod leaves the forming die.

The extruded and enamel coated rods I3 are cut into lengths suitable for drying and ring. After drying in a. suitable oven, the rods are passed through a furnace at a temperature sufficient to drive off volatile material and water Without melting the enamel coating into a glaze and Without vitrifying the jacket material.

The temperatures to which the rods have been subjected up to this part of the process are not suhciently high to cause any oxidation of the conducting material in the core, but permit volatile material to diffuse out through the ceramic jacket. After this treatment, the rods pass into a furnace at a temperature sufliciently high to vitrify and seal the materials of the protecting jacket. The enamel frit on the surface of the rods forms a glaze which seals the jacket against the admission of oxygen and protects the carbonaceous core from oxidation during the period of vitrication of the jacket and core ceramic materials. When the jacket material itself is chosen of a sufficiently low temperature vitrifying ceramic to seal the pores of the jacket almost as quickly as the rod is subjected to the elevated temperature then it is unnecessary to apply the coating of low melting point glaze. Most ceramics suitable for the jacket and having the desired mechanical strength and heat shock resistance after firing do not, however, seal rapid` ly enough at the high temperature to protect the core material from oxidation. This results in a great non-uniformity of resistance value for the nished resistor due to variations in the degree of oxidation of the core. It has, therefore, been found desirable in my process 0i manufacture of resistors to provide the above mentioned low melting glaze on the surface of the resistor. The presence of the glaze on the finished resistor also serves to protect the core from the penetration of water vapor which would cause changes in the resistance value.

As the high temperature of the furnace penetrates the core material, the ceramic contained therein is also vitrified, with the result that the core is vitrified in intimate contact with the surrounding ceramic jacket. The vitrification of the ceramic material incorporated in the core, binds the conducting particles rmly into place Whereby a permanent and stable resistance is formed.

The process so described results in the manufacture of a mechanically strong, hard ceramic rod containing a conducting core. These rods are then cut into desired lengths, and the ends are provided with terminals as heretofore described.

The above described method of making an electrical resistor element results in superior electrical characteristics of the resistance, as well as in advantages of mechanical strength and protection from outside contact and humidity. These superior electrical characteristics lie in its constant value of resistance. Many resistors made of a mixture of carbon and insulating material, depend for the resistance value obtained upon the contact resistance between adjacent particles of carbon. Such resistances manifest what is commonly called a high voltage co-efficient, whereas the voltage co-eiiicient of the resistor described herein is relatively very low. By this term, voltage co-eflicient, is meant the percentage change of resistance for a given change in voltage. In changing the voltage applied to the resistor, many of the existing types of resistances show a change of several hundred per cent in varying the voltage from a value of the order of 10 volts to a value of the order of 300 volts. In the resistors of my invention, this change of resistance with voltage is of the order of less than 10% for a similar change in voltage. My improved resistor also minimizes the socalled microphonic noise frequently occurring when composition resistors of the prior art are connected into an amplifying circuit whose output feeds a sound reproducing device. Freedom from change due to atmospheric humidity is also a characteristic of my new resistance.

The details of structure shown and described may be variously changed and modified without departing from the spirit and scope of my invention.

I claim as my invention:

1. A rod-like resistor element for electrical circuits, comprising a resistance core of electrical conducting material distributed in a vitried insulating matrix, and a rigid ceramic jacket surrounding the core and vitriiied into an integral mass with said core, said jacket and said core having substantially the same co-efiicients of thermal expansion, and shrinkage factors during vitriiication so that both vitrify at substantially the same time and without imposing internal stresses in the element whereby cooling from the vitrifying temperature and subsequent heating and cooling will not cause cracking of the element.

2. A resistor element for use in electrical circuits, comprising a. conducting path in the form of a core of resistance material, a surrounding rigid jacket of insulating material and vitriiied into an integral mass with the core, the ends of the core being adjacent the ends of the jacket, and caps enclosing the ends of the jacket and having apertures in line with the core, and lead wires having a uniform diameter no greater than the size of the apertures passing through the apertures embedded in the core material.

3. A rod-like resistor element for electrical circuits, comprising a resistance core of electrical conducting material mixed with a vitriiiable binder, and a jacket surrounding and in intimate contact with the core, said jacket comprising a vitrifiable material which is matched in its coefficient of expansion at the vitrifying temperature to that of the core and which will vitrify at approximately the same temperature as the core material, whereby the core and the jacket may be vitried into a. strong integral mass without imposing internal stresses in the element.

4. A rod-like resistor element for electrical circuits, comprising a resistance core of electrical conducting lmaterial mixed with a binder, a rigid jacket enclosing the core except at its ends, and means carried by the element for making electrical connections to the ends of the core, said jack et and said core being of materials having substantially equal thermal expansion co-efiicients at vitrifying temperatures, and substantially equal vitrifying temperatures, whereby the core and the jacket may be vitried into a strong integral mass Without imposing internal stresses in the element.

5. A resistor element for electrical circuits, comprising a resistance core and a rigid jacket surrounding the core and in intimate contact therewith, the core and the jacket being composed of materials having substantially the same vitrifying temperatures and the grain sizes of the core and the jacket materials being selected so as to render such materials substantially similar in drying and firing shrinkage and co-eiiicient of thermal expansion.

6. A resistor element for electrical circuits, comprising a resistance core of electrical conducting material and a ceramic binder, and a ceramic jacket surrounding the core and in intimate contact therewith, the conducting material being very finely divided and uniformly distributedin the core to minimize contact resistance, and the grain sizes of the ceramic materials in the core and the jacket being such as to render the shrinkage and co-eflicients of thermal expansion of such materials substantially'similar during drying and vitrification in the manufacture of the element.

7. A resistor element for use in electrical circuits comprising a conducting path of resistance material distributed in a ceramic binder and a surrounding rigid insulating ceramic body, said body being in the form of a tube with the path in the form of a core fitting within the bore of the tube, the material of the core and tube being vitrified throughout and the relative shrinkage factors thereof being such as to insure a permanent integral and homogeneous bonding therebetween upon drying and firing of the element in the manufacture thereof.

8. A rod-like resistor element composed of initially separate core and jacket members, the jacket member being of plastic ceramic insulating material and the core member being of plastic ceramic material with an electrical conducting material mixed therewith, the whole being vitried throughout, and the relative shrink age factors of the core and jacket being such as to insure a rm and permanent bonding thereof in the drying and firing process.

9. An electrical resistor element comprising a core of vitriiiable material having a conducting material mixed therewith, and a shell of vitrifiable insulating material encasing said core, the shell being composed of initially relatively finer particles than the core material, said resistor being vitried throughout.

5 10. An electrical resistor element comprising a core of vitrifiable material mixed with a conducting material encased in a shell of vvitriilable material, the materials of the core and shell being so chosen as to fix the relative drying and iiring 10 shrinkages of core and jacket so that an integral and permanent bond is formed between them upon subjection to vitrifying temperature.

11. An electrical resistor element comprising a core composed of a relatively Vcoarse grain non- 15 plastic refractory ceramic, a fine grain plastic ceramic and a conducting material, and a shell surrounding said core and composed of a relatively fine grain non-plastic refractory ceramic and a fine grain plastic ceramic, said resistor ele- 20 ment being vitriiied throughout.

12. An electrical resistor element comprising a core composed of a relatively coarse grain nonplastic refractory ceramic, a rlne grain plastic ceramic and a conducting material, and a shell surrounding said core and composed of a. relatively ne grain non-plastic refractory ceramic, a line grain plastic ceramic and'a small quantity of refractory material of good thermal conductivity, said resistor element being vitrified throughout.

13. A rod-like resistor element for electrical circuits comprising a core of non-conducting non-plastic refractory grains, a ceramic binder and finely divided carbon particles dispersed within said binder, said refractory grains being larger than the carbon particles and also larger than the particles of said ceramic binder, whereby the core is characterized by paths of comparatively low specific resistance in the interstitial spaces between the said refractory grains, and a ceramic jacket surrounding the core, said resistor being vitrliied throughout.

CERTIFICATE OF CORRECTION.

Patent No. 2,084,840.

June 22,1937.

GEORGE MILTON EHLERS.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Page 4,

second column, line 2, claim 2, after the word "apertures" insert and; and line 23, claim 4, for "temperatures" reed temperature; line 36, claim 5, for "co-efficient" read ceo-efficients: and that the said Letters PatentA should be read with these corrections therein that the same 'may conform to the record oi" the case in the Patent Office.

Signed and sealed this 24th dayof August, A. D. 1937.

(Seal) Leslie Frazer Acting Commissioner of Patents. 

